The following four types of resistors are generally used: (1) a wound resistor which is prepared by winding a Cu-Ni-base or Ni-Cr-base resistance wire around a winding core; (2) a metal coating resistor which is prepared by providing a thin film of Cr-SiO.sub.2 or Ta.sub.2 O.sub.5 on an insulator by vacuum evaporation or sputtering; (3) a thermet resistor which is prepared by mixing a glassy binder and Ru-base electrically conductive particles and then sintering the resulting mixture at elevated temperatures; and (4) a carbon resistor which is prepared by mixing a binder and carbon to prepare a paste, coating the paste on an insulator and then sintering the paste coating.
These resistors can be used satisfactorily for the usual purposes. However, they fail to satisfy the requirements described below and thus they are unsatisfactory for use in applications where these requirements are needed. Thus, a new resistor fulfilling these requirements has been desired.
Namely (1) The resistance value of such a resistor must be not less than 1M (10.sup.6).OMEGA., its size must be small, and its production cost must be low.
Specified metal coating resistors having a resistance value ranging between 1 and 30M.OMEGA. are now on the market. These resistors, however, are extremely expensive. Commercially available resistors having a resistance value exceeding 30M.OMEGA. are more expensive and, furthermore, are large-sized. Thus, these resistors are used only for specific purposes.
(2) The resistor has a complicated or specified shape and possesses the performance as a mechanic part.
A resistance value R (.OMEGA.) of a resistor, and a volume inherent resistance .rho. (.OMEGA..cm), a cross section S (cm.sup.2) and a length l (cm) of a material constituting the resistor have the following relation: EQU R=(.rho.l)/S
If it is desired to set the resistance value R at a predetermined value, it is necessary to precisely control the volume inherent resistance. It is difficult, however, to precisely control the volume inherent resistance of a material having moldability. Materials having a volume inherent resistance in the range of 1.times.10.sup.6 to 1.times.10.sup.13 .OMEGA..cm and further excellent in moldability have not been obtained.
Investigations have been made on the production of resistors having a volume inherent resistance of 1.times.10.sup.6 to 1.times.10.sup.13 .OMEGA..cm by molding a composition prepared by mixing and kneading a thermoplastic resin and electrically conductive fillers such as carbon black, carbon fibers, metal fibers and metal flakes by molding techniques such as injection molding and extrusion molding.
Resinous resistors can be molded into complicated shapes and are believed to be extremely useful as resistors functioning as mechanical parts having a high mechanical strength and so forth. In practice, however, they have not yet been put into practical use because their volume inherent resistance is quite difficult to control.
The present inventors thought that the main reason for which prior art resinous resistors have not been put into practical use was that the dispersion state of electrically conductive fillers was not precisely controlled. As a result of extensive investigations, they have found that the volume inherent resistance can be precisely controlled by using a combination of graphite and carbon black as an electrically conductive filler. Based on these findings, the present invention has been accomplished.